This patient was found to be HIV +ve man with a low CD4+ count. He underwent a biopsy of the mass. 

Histology

Microscopic Description:

Sections A1 and A2 show several small fragments of grey and white matter in which there are predominantly angiocentric, as well as diffuse, infiltrates consisting of large malignant lymphocytes. The tumor cells show high nuclear/cytoplasmic ratio, hyperchromatic nuclei, and mitotic activity. Intermixed with the large tumor cells are small, uniform in morphology, reactive-appearing lymphocytes. There are areas of tumor necrosis, as well as numerous apoptotic nuclei.

Immunohistochemistry shows strong immunoreactivity of tumor cells for CD20, consistent with B cell lymphoma. CD3-positive reactive T cells of the CD4+ phenotype are intermixed with neoplastic B lymphocytes. A large number of tumor cells show strong nuclear immunoreactivity for MIB-1 and several, although not all, tumor cells are positive for p53. Several tumor cells are immunopositive for Ebstein-Barr viral antigen and many express CD43. The CD10 and BCL6 immunohistochemistry is negative. The infiltrated neuropil shows reactive gliosis. Special stains for bacteria, fungi and acid-fast bacilli are negative.

Final Diagnosis: Large B cell lymphoma.​

The key teaching point of this case, is that in immunocompromised patients, CNS lymphoma can have variable appearances, and is often centrally non-enhancing.